When the uterus become unanchored, often because of injury during childbirth, gravity can drag it out the vagina - see below.

The arrow points to a uterus that completely protrudes from the vagina; having a baby, among other causes, can damage the tissues that hold the uterus in place and allow it to shift position. (I added the words; photo from Diseases of Women, 1922, by Dr. Harry Crossen)